Abstract The disorders collectively known as inflammatory bowel disease (IBD) including Crohn's disease and ulcerative colitis affect over 4 million people worldwide. Inappropriate accumulation of effector T cells in the intestinal mucosa plays a pivotal role in maintaining disease pathology. As such, blocking pathogenic T cell trafficking to the inflamed intestine is an area of intense investigation in IBD therapy. Recent studies have established that GPR15, a G protein coupled receptor that had been known as an HIV co-receptor, serves as a novel homing receptor that mediates T cell recruitment to colon. In the IBD mouse model where effector T cells migrate to colon and induce inflammation, genetic disruption of GPR15 in T cells was shown to prevent occurrence of colitis. In addition, most recent studies identified C10orf99, a novel chemokine that is predominantly expressed in colon, as a natural ligand that binds to and activates GPR15. These findings together highlight an emerging role of GPR15 as a critical chemokine receptor that mediates recruitment of pathogenic T cells to colon in response to C10orf99. Hence, the GPR15 is a promising new target for immune therapy of IBD. However, no reagents that can specifically modulate GPR15 activity are currently available. We propose to develop a novel peptide-based nanoparticle that inhibit ligand-induced activation of GPR15. This approach is based on a recent development of a nanoparticle forming peptide X4-2-6, a transmembrane helical peptide analog which specifically targets CXCR4 receptor for inhibition of breast cancer metastasis. The same strategy has been also applied to another chemokine receptor to develop an inhibitor peptide R321, which blocks CCR3 signaling and prevents the eosinophil recruitment to lung and airways in the mouse asthma model. Based on the well-conserved structure of chemokine receptors and the knowledge we have gained from these transmembrane peptides, we have now embarked on developing the nanoparticle inhibitor peptides containing transmembrane and extracellular loop portions of GPR15. We hypothesize that these GPR15 peptide analogs will display properties similar to X4-2-6 and R321, namely, they will form nanoparticles and inhibit the ligand-induced activation of GPR15 that is involved in pathogenic T cell recruitment to the tissues including colon. By specifically targeting GPR15 activity that plays a critical role in mediating colon homing of inflammatory cells, our study is likely to significantly impact the general approach to the development of IBD therapeutics. In Aim 1, we will use biochemical, biophysical, and molecular biology approaches to characterize the inhibitory effects of GPR15 peptides on the ligand-induced signaling in vitro, their potential for nanoparticle formation, and their mechanism of action. The peptides selected from these studies will be tested in Aim 2 for their in vivo efficacy in suppressing the colon inflammation in the clinically relevant IBD mouse model.